The present invention relates to an electroacoustic converter film used for acoustic devices such as speakers or microphones. More specifically, the present invention relates to an electroacoustic converter film which has excellent flexibility and acoustic characteristics, can stably output sound even if deformed, and makes it possible to realize a flexible speaker or the like suitably usable for a flexible display or the like. The present invention also relates to a flexible display, a vocal cord microphone and a musical instrument sensor in each of which such an electroacoustic converter film is used.
In recent years, the research related to flexible displays using a flexible substrate such as a plastic one has been progressing.
As a substrate of such flexible displays, for example, JP 2000-338901 A discloses a flexible display substrate obtained by laminating a gas barrier layer or a transparent conductive layer on a transparent plastic film.
The flexible displays are superior to the conventional displays using a glass substrate in terms of the lightweightness, slimness, flexibility, and the like, and can be placed on a curved surface of a column and the like. Moreover, since flexible displays can be stored in a rolled state, portability thereof does not deteriorate even if the screen size thereof is large. Therefore, flexible displays are drawing attention for usage in apparatuses for displaying advertisement and the like or as a display unit for personal digital assistant (PDA) devices.
When these flexible displays are used as an image display apparatus-cum-sound generation apparatus such as a television receiver that reproduces sound along with images, a speaker which is an acoustic apparatus generating sound is required.
Regarding the shape of conventional speakers, generally, there are so-called cone-type speakers having a funnel shape, dome-type speakers having a spherical shape, and the like. However, if these speakers are built in the aforementioned flexible displays, lightweightness or flexibility which is an advantage of the flexible displays may be impaired. Moreover, if the speakers are installed externally, it is inconvenient to carry the speakers, it is difficult to install the displays to a curved wall, and the external appearance may not be aesthetically pleasing.
Under these circumstances, for example, JP 2008-294493 A discloses that as a speaker, which can be integrated with a flexible display without impairing the lightweightness or flexibility, a sheet-like piezoelectric film having flexibility can be adopted.
The piezoelectric film is obtained by performing polarization processing on a uniaxially stretched poly vinylidene fluoride (PVDF) film at a high voltage, and has a property of expanding and contracting in response to an applied voltage.
In order to adopt the piezoelectric film as a speaker, the expansion and contraction movement performed along the film surface needs to be converted into vibration of the film surface. The expansion and contraction movement can be converted into vibration by holding the piezoelectric film in a curved state, and in this manner, the piezoelectric film can be caused to function as a speaker.
Incidentally, it is well known that the lowest resonance frequency f0 of a speaker diaphragm is calculated by the following equation. In the equation, s is the stiffness of a vibration system, and m is the mass.
Lowest resonance frequency:
      f    0    =            1              2        ⁢        π              ⁢                  s        m            
At this time, as the degree of bending of the piezoelectric film increases, that is, as a radius of curvature of a bending portion increases, the mechanical stiffness s decreases, hence the lowest resonance frequency f0 is reduced. That is, the sound quality (volume and frequency characteristics) of the speaker varies with the radius of curvature of the piezoelectric film.
In order to solve the above problem, in JP 2008-294493 A, the speaker has a sensor for measuring the degree of bending of the piezoelectric film, and according to the degree of bending of the piezoelectric film, sound quality is corrected by means of increasing or decreasing the amplitude by a predetermined amount for each frequency band of the audio signals, whereby stabilized sound can be output.
When a flexible display, which is integrated with a speaker formed of a piezoelectric film and has a rectangular shape in a plan view, is gripped in a gently bent manner just like documents such as a newspaper or a magazine as a portable apparatus and used by changing its screen display between portrait and landscape modes, it is preferable that the image display surface is bendable not only in the longitudinal direction but the lateral direction.
However, since the piezoelectric film formed of uniaxially stretched PVDF has in-plane anisotropy in its piezoelectric characteristics, the sound quality varies significantly with the bending direction even if the curvature is the same.
In addition, since the loss tangent of PVDF is smaller than that of the ordinary speaker diaphragm such as cone paper, resonance thereof easily becomes strong, and frequency characteristics thereof show great fluctuation of frequency. Accordingly, when the lowest resonance frequency f0 varies with the change in the curvature, the sound quality also changes to a large extent.
As described above, due to the problems intrinsic to PVDF, it is difficult for the sound quality correction means disclosed in JP 2008-294493 A described above to reproduce stabilized sound.
Meanwhile, as an example of sheet-like flexible piezoelectric materials which do not have in-plane anisotropy in their piezoelectric characteristics, there is a polymeric composite piezoelectric body obtained by dispersing a piezoelectric ceramic in a polymer matrix.
In the polymeric composite piezoelectric body, the piezoelectric ceramic is hard while the polymer matrix is soft. Therefore, there is a possibility that energy may be absorbed before vibration of the piezoelectric ceramic is transmitted over the entire piezoelectric body. This is called the transmission efficiency of mechanical vibration energy. In order to improve the transmission efficiency, the polymeric composite piezoelectric body needs to be hardened, and for doing this, the volume fraction of the piezoelectric ceramic added to the matrix needs to be at least 40% to 50% or higher.
For example, it is disclosed by Toyoki KITAYAMA in Lectures and Articles, Showa 46th Year General National Conference of the Institute of Electronics, Information and Communication Engineers, 366 (1971) that a polymeric composite piezoelectric body, which is obtained by mixing powder of PZT ceramic as a piezoelectric with PVDF by means of solvent casting or hot kneading, establishes both pliableness of PVDF and outstanding piezoelectric characteristics of PZT ceramic to some extent.
However, if the proportion of the PZT ceramic is increased to improve the piezoelectric characteristics, that is, the transmission efficiency, this results in a mechanical defect that the piezoelectric body becomes hard and brittle.
In order to solve such a problem, for example, Seiichi SHIRAI, Hiroaki NOMURA, Juro OHGA, Takeshi YAMADA and Nobuki OHGUCHI disclose an attempt at maintaining flexibility by adding fluororubber to PVDF in Technical Research Report of the Institute of Electronics, Information and Communication Engineers, 24, 15 (1980).
From the viewpoint of flexibility, this method produces a certain effect. However, generally, rubber has a Young's modulus of 1 to 10 MPa which is an extremely small value. Therefore, the addition of the rubber decreases the hardness of the polymeric composite piezoelectric body, and as a result, the transmission efficiency of vibration energy also decreases.
As described above, when the conventional polymeric composite piezoelectric body is used as a speaker diaphragm, if an attempt at imparting flexibility to the piezoelectric body is made, the energy efficiency unavoidably decreases. Therefore, the conventional polymeric composite piezoelectric body cannot produce a sufficient performance as a speaker for a flexible display.